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In-Depth Information
also show dust plumes blowing westwards towards the
South Atlantic (Eckardt, Washington and Wilkinson,
2001) from the major river channels of the Namib coast.
While aeolian processes are of considerable importance
in Australian deserts, the continent's deserts produce rel-
atively small amounts of dust. The area of greatest dust
storm frequency, as determined from meteorological sta-
tion data and TOMS AI, is the huge (1.3 million km
2
)in-
ternal drainage basin of Lake Eyre. Deflation operates on
alluvial spreads brought by the southward flowing Eyre,
Diamantina and Cooper Rivers.
Bullard
et al.
(2008) have used MODIS data to deter-
mine the spatial and temporal variability of dust emissions
from different surfaces in the Lake Eyre basin. Studying
data over the period 2003-2006, they classified 529 dust
plumes according to their source areas. Overall 37 % of
plumes originated in areas of aeolian deposits, 30 % from
alluvial deposits and floodplains and 29 % from ephemeral
lakes or playas. Sediment supply and availability rather
than erosivity were shown to be the control on deflation.
Using measurements from the Miami Aerosol Group's
Barbados station over the period 1965-1998, Prospero and
Lamb (2003) linked the great Sahel drought to increases
in North African dust production, although the cause may
relate to changes in atmospheric circulation, including
near-surface wind and long-range transport of dust by
the trade winds across the Atlantic to Barbados, rather
than simply an increase in aridity in the Sahel. Mean dust
concentrations in Barbados during the 1980s were about
four times higher (19 µg/m
3
) than the 1950s, during which
rainfall in the Sahel was some 30 % higher. In addition to
these multidecadal differences, dust output also increased
significantly in the year following El Nino events.
20.4
Future climate change
Large and significant changes to climate are projected to
take place during the Twenty-First Century as a result
of increasing greenhouse gas emissions. Many of the pa-
rameters that control dust emission, such as vegetation,
rainfall, soil moisture and surface wind speed, are ex-
pected to change. Some studies estimate the change in
atmospheric burden of dust to decrease by 20-60 % (Ma-
howald and Luo, 2003) although, in a multiclimate model
study, Tegen
et al.
(2004) argued that changes to dust
loadings are model-dependent, with projections showing
both increases and decreases globally.
There is also a research focus on both new dust sources
in the twenty-first century and change to existing key
source regions. The Met Office Hadley Centre coupled
climate model points to major Amazon forest die-back
and the emergence of this region as a key dust source late
in the twenty-first century. The driver for this change is
a reduction in rainfall (Betts, Sanderson and Woodward,
2008). In a multimodel study of the Bodele Depression,
Washington
et al.
(2009) show that the climate models
with the most realistic present-day circulation point to a
potential doubling of dust output from this key source
at the end of the twenty-first century. It is important to
point out that there are very large uncertainties associated
with these projections, beginning with the rather poorly
constrained dust emission component of the dust modules
in these models.
20.3
Temporal changes in dust
Much of this chapter has been focused on understanding
the long-term mean distribution and controls on dust. An
important additional component concerns variability and
change in dust emission, particularly given the extent of
projected climate change in the twenty-first century.
20.3.1
Observational record
Most satellite and newly implemented ground-based
aerosol remote sensing data sets such as Aeronet are still
too short to assess changes to long-term dust loadings. In-
stead we rely still on ground-based visibility observations
and measurements made by the Miami Aerosol Group.
Records show that the occurrence of dust storms was
twice more frequent during the period 1960-1984 than
during the period 1984-1997 in China. However, between
1997 and 2002, there was a significant increase in the
number of dust storms occurring during spring (Zhang
et al.
, 2003). This increase is particularly noticeable in the
deserts of northern Asia (Kurosaki and Mikami, 2005).
Moreover, large areas of ongoing desertification due to
land use have been identified in China (Xue, 1996; Zha
and Gao, 1997) and Mongolia (Natsagdorj, Jugder and
Chung, 2003) and may constitute additional dust sources.
Part of the observed changes in the case of China relate to
land use changes, including water extraction from rivers
(Gao and Washington, 2009).
20.5
Conclusions
Dust research has received a huge increase in attention
in the decade up to 2010, driven mainly by the need to
include dust in the simulation of climate change. With-
out doubt it has been the release of remote sensing data
